Field of the Invention
This invention relates to a method of increasing the recovery of oil from underground subterranean formations.
In drilling a bore into the earth, universally it has been the practice to circulate a liquid, such as water, oil, a water-in-oil emulsion, or an oil-in-water emulsion, usually with mud solids suspended therein, to and from the drilling zone during the drilling operation. Circulating drilling fluids, generally known as a drilling mud, removes drilled solids from the bit and lifts cuttings from the bore to keep the drill clean and lubricated. The drilling liquid is circulated under high pressure to assure entrapment of cuttings and expulsion of mud from the bore hole.
In newly discovered oil fields, oil will usually be recovered from a producing well under the natural pressure of the fluids present in the porous reservoir rocks (primary recovery). The naturally occurring pressure in the formation decreases as the fluids are removed and about 5% to 20% of the oil present in the formation is recovered.
Secondary recovery methods are used to recover more of the oil, such as by injecting a fluid into the reservoir to drive additional oil out of the rocks, e.g., waterflooding. Waterflooding has its own limitations as it is immiscible with oil and as the water displaces the oil, oil remaining in the reservoir reaches a limiting value known as "the residual oil saturation" and oil no longer flows. There is a strong capillary action which tends to hold the oil in the interstices of the rocks. The amount of oil recovered by secondary techniques is usually from about 5% to 30% of the oil initially present.
In recent years, more attention has been directed to enhanced recovery or tertiary recovery techniques. These tertiary recovery methods are used to recover the residual oil by overcoming the capillary forces which trap oil during waterflooding, such as by adding surfactants to the flood to decrease the interfacial tension and thus allow oil droplets to move to producing wells.
Secondary recovery of oil is also possible by the miscible fluid displacement process. Propane, for example, would be an appropriate material to utilize for it is fully miscible with oil.
The use of crude oil miscible solvents such as propane alone or in combination with kerosene to displace crude oil through a formation is well known, as, for example, in the teachings of Morse in U.S. Pat. No. 3,354,953.
Some wells have been successfully drilled at a reduced pressure using a compressed gas, such as air which is pumped into the well at the drilling site. This compressed gas flows rapidly up the well bore around the drill collar carrying with it the drilled solids, thus removing them from the drill hole. While the drilling operation is essentially a dry process, in many formations water enters the bore hole from adjacent water-containing strata or trapped underground water.
There are many advantages of the gas drilling method over the more conventional mud drilling method. However, one difficulty in mist or dry gas drilling where water seeps into the bore and accumulates in the drilling zone, is that the drilled solids tend to agglomerate as the drill rotates. These agglomerated masses become too heavy to be lifted out by the gas so that antiballing agents, and foaming agents must be introduced into the bore to prevent this condition.
The technology of air and mud drilling has been combined in an attempt to provide drilling foams which have greater lifting strength than air but which do not have the pressure limitations of drilling muds.
The rheological properties of aqueous foams are of great importance for a number of applications in petroleum production. These properties include high flow rates in tubes or pipes and in porous media such as oil-bearing sandstones. Aqueous foam has advantages in oil fields that contain viscous oil in low pressure reservoirs. In these operations, the foam raises to the surface not only sand pulverized by the bit but also pebbles and rocks of considerable size.
It is also known in the art that oil not directly recoverable by direct pumping can be displaced and recovered from a subterranean reservoir by using stimulation or secondary and tertiary recovery methods such as by injecting a drive fluid containing pressurized gas, and particularly CO.sub.2 in substantially liquid form, water and a surfactant or by fracturing the rock strata. Examples of this type of process can be found in U.S. Pat. Nos. 4,502,538 and 4,799,547.
It is noted that in the CO.sub.2 drive system, use is made of a surfactant to form a "foam" which reduces the mobility of the CO.sub.2 in the reservoir. It is well recognized in the art that many reservoir flood or drive processes, including those utilizing CO.sub.2, suffer from a tendency of injected fluids to sweep oil from only a limited area of the reservoir. The fluids break through to the recovery well before they have the opportunity to efficiently invade and displace oil from the reservoir rock.
The requirements for utilization of an aqueous foam in subterranean formations include high stability with waters containing quantities of soluble salts, such as sodium chloride, calcium salts and/or magnesium salts and capability for handling a variety of foam breaking elements, e.g., hydrocarbon tolerance from such as crude oil and solids. Further, the foam must not degrade under extremes of the physical environment of use such as drilling pressure temperature, earth particles and the like.
In addition, the most important performance characteristics of a surfactant used in a CO.sub.2 drive process is its ability to maintain excellent foam generation and stability properties and thus maintain control over mobility of the CO.sub.2 in the presence of a wide variety of crude oils. The surfactant used in the CO.sub.2 drive process should be substantially less sensitive to compositional variations and changes in the crude oil deposit as this can seriously affect the foam holding characteristics of the surfactant.
Historically, alcohol ether sulfates have been used for CO.sub.2 mobility control agents. Alcohol ether sulfates provide an overall cost advantage because of a production increase.
It is an object of this invention to proved a surfactant composition having excellent foaming power and high tolerance for aqueous solutions containing electrolytes and/or hydrocarbons for use in oil well bores, such as for drilling, stimulation and secondary and tertiary oil recovery.
It is now been found that improved foam control can be obtained utilizing a specified group of amphoteric surfactants.